The present invention relates generally to gas lasers, and more particularly to a composition for structural applications in a gas laser system using hydrogen fluoride as the lasing medium.
Improvements in lasers have been significant and include gas laser systems which are capable of providing extremely high-power radiation outputs. In the operation of a gas laser a chemical reaction is initiated between the molecules of an exciting gas and the vibrationally excitable molecules of a second gas to generate coherent electromagnetic radiation in the optical frequency range. This coherent radiation emission is effected by the population inversion established between upper and lower energy levels of the gaseous lasing mediums. The gas lasing system is rendered functional by coupling the excited molecules out of the reaction chamber or oscillating cavity to a point of use. In a gas laser this coupling can be effected by rapidly expanding the vibrationally excited gas or gas mixture through two-dimensional, i.e., converging-diverging nozzles. By freely expanding the vibrationally excited gas through the nozzles the flow of the excited gases entering the nozzle is sequentially compressed, attains sonic velocity near the nozzle throat and then freely expands to supersonic velocities downstream of the throat to produce the lasing action.
In a hydrogen-fluoride laser the lasing action is achieved by vibrating hydrogen-fluoride molecules to a highly excited state by an exothermic chemical reaction between the hydrogen or deuterium and fluorine in the presence of helium. These excited gases like the prior art gases are directed out of the reaction chamber through a converging-diverging nozzle to attain the supersonic velocities necessary to provide the lasing action. The lasing action provided by the hydrogen-fluoride molecules is particularly desirable because the content radiation wavelength has a low absorptivity by water molecules so that the laser beam would be minimally affected by atmospheric moisture.
While hydrogen-fluoride lasers are advantageous for the above reasons, a major problem with such lasers has been due to the lack of a structural material capable of withstanding the highly corrosive fluorine-containing gaseous environments especially at elevated temperatures. Another problem is due to the extensive thermal shock and stress encountered by the laser nozzles. At surface temperatures greater than about 1300.degree. K. the various known structural materials proved to be ineffective for use in hydrogen-fluoride lasing systems. For example, in the prior art nickel had been utilized as the principal material for the construction of hydrogen-fluoride laser nozzles. A layer of nickel fluoride is formed on the nozzle surface when exposed to the fluorine-containing gases. This nickel fluoride layer is relatively passive in the fluorine environments at surface temperatures less than about 1300.degree. K. but rapidly sublimes at greater temperatures with this rate of attack upon the nickel increasing with increasing temperatures. Accordingly, the nickel-metal nozzles can be protected from corrosion by maintaining the nozzle surface temperatures at less than 1300.degree. K. with an internal coolant arrangement. While cooling the nozzle effectively protects the nozzle, the efficiency of the laser is significantly decreased since a large amount of the laser energy is transferred to the coolant so as to significantly decrease the number of molecules in the excited state.
Previous work also included the fabrication of simulated nozzles from lanthanum hexaboride (LaB.sub.6) for testing in a hydrogen-fluoride flame. The lanthanum-hexaboride composition becomes coated with an adherent layer of lanthanum-trifluoride (LaF.sub.3) when contacted by the flame at elevated temperatures. Corrosion of the lanthanum hexaboride by the fluorine-containing hydrogen fluoride gas (typically 2HF, F.sub.2) at surface temperatures greater than 1300.degree. K. and up to about 1800.degree. K. were essentially obviated by this layer of lanthanum trifluoride. While this nozzle-forming composition provides a significant improvement over the materials of the prior art as a nozzle constructing material for hydrogen-fluoride lasers, complete success was not achieved with lanthanum hexaboride since it is extremely brittle and very difficult to machine. Further, due to the relatively poor thermal shock resistance a hydrogen-fluoride simulated laser nozzle formed from lanthanum hexaboride underwent deleterious stressing and cracking during the operation at surface temperatures greater than about 1400.degree. K. Thus, because of the difficulty in machining lanthanum hexaboride and its relatively poor thermal shock resistance, flexibility in the design of laser nozzles, and other laser hardware requiring the material is very limited.